slake
To slake is to lessen or abate. You can slake your thirst by drinking, or slake someone's fear by reassuring them. To slake can also be to cool or refresh by wetting or moistening, and to slake lime (calcium oxide) is to change it into calcium hydroxide (slaked lime) by combining it with water. Of all these meanings, the oldest is the first one. The word is from Middle English slaken (to abate), from Old English slacean, derived from slaec (sluggish). This root was also the source of Modern English slack, slacken, and slacker. From the idea of slaking thirst by drinking came slaking by adding water to cool or refresh. That usage led to slaking as a name for the chemical alteration of lime by adding water. Unlike slaking to cool, slaking lime causes the release of heat.